An approximate mathematical formula used to compare and contrast the size of a woman's breasts to the size of her ass. Typically if one is bigger than the other, the woman in question has a skewed or poor tits- to- ass ratio - If the two are relatively proportional, then she has an ideal tits- to- ass ratio. In some cases, the formula can be trivialized when the woman's good looks outweighs both of those factors.
"Sarah's got them titties tho"
"Yeah but her Rebello's ratio is all off, man